In electron cyclotron resonance sources, the ions are obtained by ionisation of the particles of a gaseous medium formed by one or more gases, metal vapours or molecules in vapour phase, contained in an axially symmetrical sealed enclosure, by means of a plasma of electrons highly accelerated by electron cyclotron resonance.
Electron cyclotron resonance is obtained due to the combined action of a high-frequency (HF) electromagnetic wave injected into the enclosure and of a magnetic field of which the modulus structure corresponds to a structure of the magnetic mirror type, referred to as a minimum B structure. The profile of the magnetic mirror structure has at least two maxima (Bmax) on the abscissas, arranged respectively in the regions of injection and extraction of the source, and a minimum (Bmin) arranged between the two maxima (Bmax).
The two maxima (Bmax) have a value greater than the value of the magnetic field (Bres) for which electron cyclotron resonance satisfying the condition Bres=f·2πm/e is achieved, where e represents the electron charge, m represents the electron mass, and f represents the frequency of the HF electromagnetic wave.
The minimum (Bmin) has a value equal to or less than the value for which electron cyclotron resonance is achieved.
Waveguide-type electron cyclotron resonance sources of multicharged ions, such as the source described in patent EP 0527082, are known.
In patent EP 0527082, the introduction of the high-frequency electromagnetic wave can be ensured, both by a coaxial transition and by direct injection, from rectangular or circular fundamental mode waveguides. According to the described invention, the enclosure, in its mid-plane, has a cross section substantially equal to that of the waveguide ensuring the injection of the electromagnetic field into the enclosure and the propagation of the wave in the enclosure referred to as a waveguide enclosure.
The use of the enclosure as a waveguide enables the propagation of the HF wave in any confinement enclosure and thus the formation of a plasma at the place where the ECR conditions are combined.
Patent EP 0527082 also proposes the use of a specific arrangement of axially symmetrical permanent magnets, making it possible to avoid the use of solenoids and making it possible to produce a simple source of small size.
However, the use of these ion sources requires the injection of a gas or of a metal vapour into the confinement enclosure in order to initiate and maintain the electron cyclotron resonance plasma. The gas has to be injected into the enclosure under conditions that make it possible to ensure a minimum pressure of approximately 10−4 mbar in the confinement enclosure in order to ensure the ignition of the plasma.
The use of this type of ion source thus results in the need for control and adjustment of the pressure in the confinement enclosure before the injection of the gas so as to achieve the pressure required for the ignition of the plasma.